Integrated Membrane Separation Processes for Recycling of Valuable Wastewater Streams: Nanofiltration, Membrane Distillation, and Membrane Crystallizers Revisited

One of the suggestions made by Enrico Drioli to the membrane community for many years is to work on a radical change from conventional separation and conversion methods to membrane-based separation methods, making use of integrated systems combining a range of membrane separation processes. This paper looks back at these ideas, taking a joint paper written by myself with Efrem Curcio and Enrico Drioli and published in 2004, as a reference. Three processes were central in this paper: the use of nanofiltration for fractionation, membrane distillation, and membrane contactors combined with crystallization as membrane crystallizers. The objective of this paper is to monitor the progress in these three aspects of the proposed overall integrated process. This is done based on the literature and our own expertise. The intention is not to focus on a new and limited subset of results but to evaluate the overall idea of process integration one decade after being proposed. Based on this, it is concluded that the required fractionation in nanofiltration appears to be possible by process engineering rather than membrane engineering. Membrane distillation is today in a clear exponential growth phase and has emerged from research laboratories into larger scale applications, although most still in desalination whereas the potential is larger. Membrane crystallization remains an undiscovered process for many, in spite of its proven technical performance and interest in the scientific literature.